Cumulate Thermobarometry

小安的列斯火山弧为活跃的岛弧俯冲带提供了一个异常完整和容易获得的案例研究，该俯冲带是由至少始新世以来美国板块俯冲到加勒比海板块东缘下方形成的。该弧提供了一个世界级的机会，可以在相对简单的大洋内环境中探索俯冲带过程。弧岩浆作用过程发生在深部，必须通过地表可获得的信息来推断。这些信息的一个主要来源是岩浆岩的性质和组成，岩浆岩可能是作为火山熔岩喷发出来的，或者在火山管道系统内结晶，由上升的岩浆带到地表，然后作为内部的捕虏体喷发。熔岩和包体都反映了俯冲板块上方地幔楔形中富含挥发分的岩浆熔融和地表熔岩喷发之间发生的过程的各个方面。一类重要的捕虏体，称为堆积岩，是由晶体形成的，这些晶体在冷却的岩浆中生长，然后下沉并聚集在岩浆室的底部，从而表明岩浆储存在火山系统中的离散位置。作为这些捕虏体母体的累积序列位于地壳中一个未知的深度，但必须包含大量的信息，这些信息编码在它们的晶体和捕获的熔体气泡的组成中。这些信息包括关于堆积物结晶的岩浆来源的线索，以及它在储存和运输过程中与其他岩浆的混合。目前，调查人员必须根据堆积包体的数量做出推断，而不能对弧下深层的母体堆积序列或各种堆积样品之间的相互关系有一个连贯的了解。在这项研究中，我们将试图对埋藏的累积序列的横向分布和深度分布有一个连贯的了解。Pi和他的同事以前的工作详细说明了沿弧形喷发的堆积包体的矿物学、结构外观和成分的变化；这些被解释为表明，在弧形以北，为个别火山提供燃料的岩浆室往往位于较浅的水平，岩浆含有较高比例的水。在沿弧长750公里的累积捕捉体取样方面，仍然存在着小但重要的差距，我们建议纠正这些差距。需要一种新的方法来从包体中的矿物成分中得出精确的累积结晶深度和温度。我们将利用与观察到的堆积包体相似的产品进行的大型实验数据库；我们将通过额外的实验来补充，以确保在可控的压力、温度、挥发含量和氧逸度条件下获得完整的累积成分和矿物组成范围。然后我们将根据数据库校准热力学模型，代表每个观察到的矿物相。这些模型将描述平衡状态下各矿物相组成随控制条件的变化。将对它们进行校准，以期通过严格的统计分析恢复实验条件。然后，完成的模型将应用于自然样品数据库，目的是根据到达地表的样品，建立地下岩浆存储系统的近似三维图像。原则上，这一信息可以与地震成像结合起来，并与之对照进行核实，地震成像利用地震活动来探测地壳结构，也可以与辐射测年结合，为分析提供时间维度。从这个项目中得出的模型和解释将与世界各地的许多其他俯冲带环境相关。

The Lesser Antilles volcanic arc provides an unusually complete and accessible case study of an active island arc subduction zone, formed by the subduction of the American plate under the eastern edge of the Caribbean plate since at least the Eocene. The arc offers a worldclass opportunity to explore subduction zone processes in a relatively simple intraoceanic setting. Arc-magmatism processes take place at depth and must be inferred through information available at the surface. A major source of such information is the nature and composition of magmatic rocks, which may either be erupted as lava from the volcanoes, or crystallized within the volcanic plumbing system, carried to the surface by ascending magma, and erupted as xenoliths within it. Both lavas and xenoliths reflect aspects of the processes that take place between the melting of volatile-rich magma in the mantle wedge above the subducting plate, and the eruption of lavas on the surface. An important category of xenoliths, known as cumulates, are formed from crystals that grew in a cooling magma, sank and accumulated at the bottom of the magma chamber, thereby indicating magma storage at discrete locations in the volcanic system. The cumulate sequences that are parental to these xenoliths lie at an unknown depth in the Earth's crust, but must contain extensive information encoded in the compositions of their crystals and captured bubbles of melt. This information includes hints about the source of the magma from which the cumulates crystallized, and its mixing with other magmas during storage and transport. Investigators are currently obliged to make inferences based on the population of cumulate xenoliths, without having a coherent picture of the parental cumulate sequences at depth under the arc, or the relationships of the various cumulate samples to each other. In this study we will attempt to gain a coherent understanding of distribution of buried cumulate sequences, both laterally along the arc and at depth. Previous work by the PI and co-workers has detailed the variation in the mineralogy, textural appearance and composition of cumulate xenoliths erupted along the arc; these are interpreted as showing that, towards the north of the arc, the magma chambers feeding the individual volcanoes tend to lie at shallower levels, and the magmas contain a higher proportion of water. Small but important gaps remain in sampling of cumulate xenoliths along the 750km length of the arc, and we propose to rectify these. A new method is required to derive precise depths and temperatures of cumulate crystallization from compositions of minerals in the xenoliths. We will make use of a large database of experiments with products that approximately mimic the observed cumulate xenoliths; we will supplement it with additional experiments, to ensure that the full observed range of cumulate compositions and mineralogies has been obtained under controlled conditions of pressure, temperature, volatile content and oxygen fugacity. We will then calibrate thermodynamic models against the database, representing each of the observed mineral phases. The models will characterize the variation of composition with the controlled conditions among mineral phases in equilibrium. They will be calibrated with a view to being able to recover the experimental conditions via a rigorous statistical analysis. The finished models will then be applied to the database of natural samples, with the aim of building up an approximate three-dimensional picture of the subsurface magma storage system based on the samples that reach the surface. In principle this information may be combined with and verified against seismic imaging, which uses seismicity to probe crustal structure, and also with radiometric dating to provide a time-dimension to the analysis. The models and interpretations derived from this project will be relevant to a number of other subduction zone settings worldwide.